This invention relates to a rectification system for the production of ethane and higher hydrocarbons from high pressure, hydrocarbon-containing feed gas in which the feed gas is cooled and expanded in several stages and the resulting condensates are fed to a rectifying column to provide an overhead product consisting essentially of methane and a bottom product consisting essentially of ethane and higher hydrocarbons.
There is a known process of treating natural gas or refinery gas, containing methane, ethane, propane and higher hydrocarbons, comprising the steps of freeing the gas of acid gases, such as H.sub.2 S and CO.sub.2, that are possibly present, and from water; cooling the resultant gas by external refrigeration and heat exchange with itself or fractions thereof in several cooling and phase separation stages; expanding the resulting condensates into a rectifying column at points that match the equilibrium compositions in the rectifying column; and work-expanding the gas phase of the last cooling stage in an expansion engine, e.g., a turbine. In the rectifying column, methane and small amounts of ethane are recovered as overhead gas at the top of the column, and a practically methane-free C.sub.2+ fraction is obtained as the bottoms product. The gases resulting from the last expansion are recompressed and delivered as product gas (e.g., U.S. Pat. No. 4,061,481).
Frequently, the incoming gases are at various pressure levels. For example, crude oil mixed with gas is usually produced during the multistage expansion and subsequent stabilization of the crude oil. In this case, the gaseous fractions at various pressure levels have been brought to a single high pressure, dependent on the composition and purity of the products to be obtained, mostly around 60 to 70 bars, and then the further processing of the gaseous mixture is conducted at this pressure.
This known technique is extremely costly in energy. The fractions under low pressure contain mostly higher and thus heavier hydrocarbons, whose compression requires a considerable expenditure of energy. Moreover, in compression at high pressure, condensates result which contain water and possibly also acid gases and thus cannot be fed directly to fractionation.